1 . Field of the Invention
This invention relates generally to determining a reactor core design for a nuclear reactor which has a desired minimum control blade definition.
2 . Description of Related Art
A core of a light-water power nuclear reactor (LWR) such as boiling water reactor (BWR) or pressurized water reactor (PWR) has several hundred individual fuel bundles (fuel assemblies) of fuel rods (BWR) or groups of fuel rods (PWR) that have different characteristics. These bundles (fuel rod groups) are preferably arranged so that interaction between rods within a fuel bundle (rod group), and between fuel bundles (fuel rod groups) satisfies all regulatory and reactor design constraints, including governmental and customer-specified constraints. Additionally, the core design must be determined so as to optimize core cycle energy. Core cycle energy is the amount of energy that a reactor core generates before the core needs to be reloaded with new fuel elements, such as is done at an outage. A core energy cycle (or fuel cycle) in a power reactor such as a BWR may typically extend 12 months, 18 months, 2 years or more before the reactor is shutdown at a planned maintenance outage for replacement of fresh fuel bundles and shifting of exposed fuel bundles (burnt, twice-burnt, etc.) to
In the case of a BWR, for example, the number of potential bundle arrangements within the core and individual fuel element arrangements within a bundle may be in excess of several hundred factorial. From these many different possible configurations, only a small percentage of core designs may satisfy all applicable design constraints. Further, only a small percentage of these core designs, which do satisfy all applicable design constraints, are economical.
In addition to determining the fresh fuel arrangement of the core design, control blade operational strategy should be evaluated in the design process. Conventional control blade operational strategy involves determination of the blade groups, blade positions, and blade sequence intervals during the cycle. The control blade operational strategy may be performed in conjunction with the core loading pattern design (core design) as part of the fuel cycle design process. A typical operational strategy may involve the use of multiple blade groups that are periodically exchanged during the cycle as a means of controlling the power shape (i.e. margins to thermal limits) as well as the hot excess reactivity.
Control blades in BWRs contain neutron-absorbing material that “damp” the nuclear reactions (and thus local power) in the proximity of the control blade when inserted into the core. Typically in a BWR, a control bladed travels in what is called a fuel channel between adjacent fuel bundles. Inserting a control blade thus decreases the core reactivity while removing the blade increases the core reactivity. Control blades “wear out” over time (i.e., the blades' absorbing capability diminishes) proportional to the usage and must be periodically replaced. Replacement of control blades involves added cost to the utility both in terms of blade purchase as well as the reactor down time (typically during a refueling outage) required to perform the control blade maintenance.
A recent phenomenon involving use of control blades in BWRs has been observed, known as a “shadow corrosion” mechanism or event. A shadow corrosion mechanism or event may occur when a control blade is inserted for extended periods of time in the proximity of fresh fuel bundles. The shadow corrosion mechanism causes hydrogen pickup in the fuel channel that results in the channel bowing later in life after an extended period of irradiation. This channel bow effect can interfere with the ability of the blade to be inserted, creating a potential safety issue since operable control blades are a requirement for shutting down the reactor in an emergency situation. Replacement of those channels exhibiting shadow corrosion phenomenon partway through the life of a fuel bundle is costly, both in terms of reactor down time and the purchase cost of the channel.
Further, individual control blade movements or blade sequence exchanges, in general, place stresses on the nuclear fuel due the effect on local power in the proximity of the blades being moved. Various “soft handling” guidelines (i.e. recommendations provided by the fuel vendor) exist for performing such blade movements. Often this involves a reduction in reactor power level as a means of reducing the stresses placed on the fuel. This impacts the overall capacity factor of the plant.
Use of control blades is integral to the design of the core loading pattern design and operational strategy. To address blade “wear out”, blade management strategies are employed in which high duty blades are shuffled or swapped with low duty blades. Still, the maintenance times required are expensive with the necessity to nevertheless replace control blades. Similarly, the channel bow mitigation requires either re-channeling of fuel midway through life or alternatively, modification of the core loading pattern design to shuffle susceptible fuel to uncontrolled locations. Modification of the core loading design to mitigate channel bow places additional constraints on the core loading design with the potential for a large economic penalty in terms of having to purchase of additional fresh fuel bundles. In addition, the aforementioned, “soft handling” guidelines as well as operational rules are in place regarding control blade movement.
Traditionally, core design determinations have been made on a trial and error basis. Specifically, and based on only the past experience of the engineer or designer, in designing a core design an initial core design was identified. The initially identified design was then simulated in a computer. If a particular design constraint was not satisfied, then the arrangement was modified and another computer simulation was run. Many weeks of resources typically were required before an appropriate core design was identified using the above-described procedure.
For example, a conventional process used is a stand-alone manual design process that requires a designer to repeatedly enter reactor plant specific operational parameters into an ASCII text file, which is an input file. Data entered into the input file includes control blade notch positions of control blades (if the evaluated reactor is a boiling water reactor (BWR)), core flow, core exposure (e.g., the amount of burn in a core energy cycle, measured in mega-watt days per short ton (MWD/st), etc.
A Nuclear Regulatory Commission (NRC) licensed core simulation program reads the resulting input file and outputs the results of the simulation to a text or binary file. A designer then evaluates the simulation output to determine if the design criteria have been met, and also to verify that no violations of margins to thermal limits have occurred. Failure to meet design criteria (i.e., violations of one or more limits) require a manual designer modification to the input file. Specifically, the designer would manually change one or more operation parameter and rerun the core simulation program. This process is repeated until a satisfactory core loading pattern is achieved.
This process is extremely time consuming. The required ASCII text files are laborious to construct, and often are error prone. The files are fixed-format and extremely long, sometimes exceeding five thousand or more lines of code. A single error in the file results in a crash of the simulator, or worse, results in a mildly errant result that may be hard to initially detect, but will profligate with time and iterations to perhaps reduce core cycle energy when placed in an actual operating nuclear reactor core.
Further, no assistance is provided via the manual iterative process in order to guide a designer toward a more favorable core loading pattern design solution. In the current process, the responsible designer or engineer's experience and intuition are the sole means of determining a core design solution. Moreover, conventional core design processes to determine a desired core loading pattern design to be implemented for a given energy cycle (or cycles) have been inadequate in mitigating the channel bow problems due to the shadow corrosion phenomenon, or in reducing control blade movement so as to minimize blade wear.